1. Field of the Invention
Circuit breakers and motor safety switches are devices which are designed to protect the supply leads or the motor from excess current consumption and hence thermal overload.
In safety devices of this nature, a distinction is drawn between two operating situations. One operating situation is the low-resistance short circuit, in which extremely high currents occur momentarily. These extremely high currents must be cut out very quickly, to which end electromagnetic drives are used. The current from the supply lead or to the motor which is to be protected flows through the magnet winding of an electromagnet which causes an armature to move when a critical current limit value is exceeded, as a result of which the switch contacts are opened.
The other operating situation, which is also intended to lead to a cutout, is where the current rating is exceeded slightly, by comparison with the short circuit, beginning at approximately 20%. The speed at which such an excess current is to be cut out depends on the extent to which the rating is exceeded. The trip time fluctuates between minutes and milliseconds.
Brief and minor excess currents often occur, in particular, for example, when switching on motors and the like, and it is not desirable for this to trip the safety device immediately. On the other hand, it is desirable to suppress the situation where the current rating is exceeded for long periods.
In order to achieve both the time delay and the detection of a current load which lies about 20% above the rating, bimetallic devices which are heated by means of the current flowing through the circuit breaker or the motor safety switch are used in the motor safety switches and in the circuit breakers. These bimetallic devices have, so to speak, an integrating characteristic, in that they detect how long a correspondingly high overcurrent flows for. In doing so, it is necessary to ensure that in the event of the rated current being exceeded considerably but at a level which lies well below that of a short circuit, a relatively quick cutout will be achieved by means of the bimetal component.
With the aid of the bimetal component, it is necessary to cover a current range which begins at approximately 20% above the current rating and extends as far as the lowest current level at which the magnet drive device can become active. The intention is that a very short reaction time of the bimetal component is to be achieved at high current levels.
2. Description of the Related Art
DE 36 37 275 C1 has disclosed an overcurrent trip for safety switches which has a comparatively short reaction time when the rated current is considerably exceeded. The arrangement comprises an iron yoke which is bent in the form of a U and the two limbs of which contain mutually aligned bores. These bores hold an aluminum tube which serves to support a magnet winding which is arranged thereon and is situated between the limbs of the iron yoke. An iron core with a continuous bore, in which a ram is displaceably guided, rests immovably inside the tube. One end of the rod bears against an armature which is also situated in the tube and is held at a distance from the adjoining end side of the iron core with the aid of a helical compression spring.
On one side, the tube merges into a cup-like attachment which is provided with a concave base. This concave base continues into the external end side of the iron core.
In the cup-shaped extension there is a circular bimetal snap-action disk which, at the edge side, is pressed against the base with the aid of a cover which is fitted into the cup. In order to create as good a level of heat transfer as possible, the radius of curvature of the cold bimetal snap-action disk corresponds to the radius of curvature of the base of the cup.
This known arrangement is suitable for monitoring both the short circuit situation and the overcurrent situation. The consumer current flows across the winding and, in the event of a short circuit, a magnetic field is generated which moves the armature towards the fixed iron core, counter to the action of the spring, with the result that the rod is pushed forward and the latching of the trip mechanism of the switch is unlocked.
At the same time, the winding on the aluminum tube serves as a heating winding for the bimetal disk. In the event of an overcurrent which lies above the rating but below the short circuit current, the winding heats the entire magnet yoke until the snap-over temperature of the bimetal snap-action disk is reached. The snap-action disk suddenly snaps over to the opposite direction, thus also pushing the rod toward the trip mechanism, specifically with the aid of an annular shoulder which is arranged on the rod.
This solution, which has proved to be an excellent solution for low rated currents, exhibits problems when used for high rated currents, because the reaction time of the thermal tripping is too long. The heat absorption capacity of the iron yoke, the aluminum tube and the cup is so high that the reaction time is considerably delayed when high overcurrents occur. Moreover, the heat transfer to the snap-action disk is poor. Even at low excess temperatures, the disk begins to creep and to lift off the base of the cup in its central region, with the result that an air gap which impairs the heat transfer is formed between the disk and the base. The heat is only introduced from the edge of the disk, with the result that, combined with the thermal inertia of the magnet device, a reaction performance which is unsuitable in terms of time is produced.
CH 319 008 has disclosed a bimetal snap-action switch which operates with a curved bimetal disk. The housing of the bimetal switch has a base which projects convexly inward and against which the concave side of the bimetal disk bears. A fastening rivet which fixes the bimetal disk to the convex housing base passes through the center of the bimetal disk. The aim of this design is to produce a good level of thermal contact between the bimetal disk and the housing in the at-rest position corresponding to the cold state.